With ever increasing reliance on electronic mail as a method of communication, there is a need for improving security and reliability of e-mail transmissions. To accomplish this task, various authentication and confidentiality techniques have been developed. Authentication is a procedure that allows communicating parties to verify that contents of a message have not been altered and that the source is authentic. In turn, confidentiality services provide protection of message content from release to unauthorized parties. When combined, the authentication and confidentiality services protect electronic transmissions from various passive security attacks, such as eavesdropping and traffic monitoring, as well as active attacks, such as impersonation and falsification of data.
One implementation of these security services is a Public Key Infrastructure (PKI), which employs an asymmetric encryption technique for encrypting documents and messages. In a PKI scheme, each user generates a pair of keys: a public key and a private key. The public key is placed in a public register, while the private key is kept private. To communicate, a message is encrypted with one of the keys, so that it can be decrypted only with the other key. To assure authenticity of public keys, PKI provides for a Certification Authority (CA), which is a trusted third party that is authorized to issue certificates consisting of a public key and an ID of the key owner. Thus, the authenticity of a public key can be checked by referring to the associated public-key certificate.
In general, the PGP and S/MIME techniques are quite effective in providing both the authentication and confidentiality to electronic message transmissions; there are, however, instances in which both security schemes can be compromised. In one example, a malicious third party may use a forged public key to impersonate another by sending messages signed with a forged digital signature. In another example, a third party may forge both the certificate and the pair of keys with credentials of another and send e-mail messages signed with a forged digital signature. The recipient of such a message may believe that the message is authentic because the key used to decrypt it will be validated by the certificate. There are other examples of impersonation and various e-mail security attacks that S/MIME, PGP and other asymmetric or symmetric security schemes fail to prevent.